The present invention relates to a method for setting the amount of light in a light-emitting element array, particularly to a method for setting the amount of light for a light-emitting element array in which light-emitting thyristors are arrayed, each thyristor having a current-light output characteristic in which a luminous efficiency is decreased in a lower current field.
A light-emitting diode (LED) is generally used for a light-emitting element array in an optical print head an optical printer. In a light-emitting element array using LEDs, an array pitch of LEDs is determined by a critical pitch of wire bonding method, i.e. 500 dpi (dots per inch). Therefore, it is impossible to increase a resolution of a light-emitting element array by arraying LEDs at high density.
In order to resolve this problem, the applicant has already proposed a light-emitting element array using a three-terminal light-emitting thyristor of pnpn-structure, to which Japanese Patent has been issued (Japanese Patent No.2807910) that is hereby incorporated by reference.
According to this patent, an array of light-emitting elements is divided into blocks n by n (n is an integerxe2x89xa72), the gates of n light-emitting thyristors included in each block are separately connected to n lines, and the anodes or cathodes of n light-emitting elements included in each block are commonly connected to one electrode, respectively. In this manner, the number of electrodes to supply signals for light emission may be decreased, so that an array pitch of light-emitting elements becomes smaller.
An I-L (current-light output) characteristic of a three-terminal light-emitting thyristor is not a straight line passing through an origin of orthogonal coordinates. FIG. 1 shows an I-L characteristic of a three-terminal light-emitting thyristor which has an area of light-emitting portion of 20 xcexcmxc3x9720 xcexcm. An abscissa designates a current (mA), and an ordinate a light output (xcexcW). The I-L characteristic varies linearly in the field larger than 10 mA and light is emitted, but light is not substantially emitted in the field smaller than 5 mA. Therefore, a luminous efficiency becomes lower when a light-emitting thyristor is used in a smaller current field, and then a power consumption for obtaining a required exposure energy is increased, resulting in the temperature increasing of an optical print head.
On the other hand, when a current density in a light-emitting portion of the thyristor is increased, the reduction of the amount of light due to current supply becomes extremely larger. The reduction of the amount of light is proportional to the time duration of current supply and is increased exponentially with respect to the current density. FIG. 2 shows the variation of the amount of light due to the time duration of current supply. An abscissa designates time (Hour), and an ordinate the reduction (%) of the amount of light from an initial value.
Referring to FIG. 3, there is shown a graph designating the reduction of the amount of light after 1000 hours of current supply, which is obtained from the result shown in the graph of FIG. 2. It is recognized from FIG. 3 that a current density should be smaller than 100 MA/m2 in order to hold the decrease of the amount of light within 2%. It has also been known that the current density such as 100 MA/m2 is an upper limit in LED for communication, and lattice defects are extremely increased at a current density more than 100 MA/m2. Therefore, it is desirable that the density of a current supplied to a light-emitting thyristor is smaller than 100 MA/m2. In the case of a three-terminal light-emitting thyristor having a light-emitting portion area of 20 xcexcmxc3x9720 xcexcm, a current through the light-emitting portion corresponds to 40 mA when a current density is 100 MA/m2.
Consequently, it is required that the amount of light emitted from a light-emitting thyristor is set so that a predetermined exposure energy may be obtained without decreasing a luminous efficiency of a light-emitting thyristor.
The object of the present invention is to provide a method for setting the amount of light for an array in which a plurality of light-emitting elements such as light-emitting thyristors are arrayed, each light-emitting element or thyristor having an I-L characteristic which is not a straight line passing through an origin of orthogonal coordinates, the method being adapted so that a predetermined exposure energy may be obtained without decreasing a luminous efficiency.
According to the present invention, in a light-emitting thyristor having an I-L characteristic in which a luminous efficiency is decreased in a lower current field, the density D of a current to be supplied to the light-emitting is selected so as to satisfy the range of 3xc3x97Dth less than D less than 100 MA/m2, wherein Dth is a threshold current density for light emission which is defined as a current density corresponding to the value of a current at a point where a tangent drawn at the value of a current corresponding to a current density of 500 MA/m2 with respect to the curve of the I-L characteristic intersects a current axis of a graph designating the I-L characteristic.
When an exposure energy is regulated in a multiple gradation manner, the graduation of the exposure energy is regulated by modulating the time duration of light emission of the thyristor, or both of the value of a current and the time duration of light emission of the thyristor.